What is growing on my young bur oak?

Asked August 12, 2017, 8:53 AM EDT

Earlier this summer I picked off many small black galls or growths on the branches of my oak in southern Mn. They were about 1/2" in diameter. They were firmly attached. Now these cotton like growths have appeared. I suspect they are related. Can you tell me what this is and what I can do about it. See picture. Thanks